The present invention relates to apparatus for cooling particulate material such as lime, cement, lightweight aggregate, ores and the like which have calcined or sintered in a rotary kiln or other pyroprocessing system.
It is known to provide a stationary contact cooler for containing hot particulate material discharged from a rotary kiln within an insulated, upright shell while supplying cooling air thereto by an appropriately dimensioned blower. After passing through the material, this cooling air is either vented to the atmosphere or used as combustion air for the calcining or sintering process. Apparatus of this type is disclosed, for example in the U.S. Pat. No. 3,274,701 to Gerhard Niemitz which is assigned to the assignee of the present invention.
The various types of contact cooling apparatus known in the art have disadvantages which limit the quality and efficiency of cooling.
When particles of heated material are fed into the top of the cooling apparatus, for example from a rotary kiln, they do not necessarily fall symmetrically about the central axis of the shell. Many operating variables, such as the particle size distribution, the discharge configuration of the rotary kiln, the capacity, and rotational speed of the rotary kiln, affect the point of entry of the hot particles into the shell. These parameters cannot be predicted with sufficient accuracy to properly locate the cooling apparatus relative to the rotary kiln before the unit is put into operation.
With the contact coolers known in the art, it has been assumed that the material particles are discharged from the kiln without segregation of material by particle size into the center of the cooling bed. One can observe from operating kilns, particularly large diameter kilns, that the finer particles discharge farther off of cooler center in the direction of the rotating kiln, thus creating regions in the cooler containing high concentration of fines. Since the finer particles offer a greater resistance to the flow of cooling air and air follows the path of least resistance, the air passes through the region of the cooler which has the coarser particles. As a result, maximum cooling efficiency is not achieved and the particles from the region containing high concentration of fine are discharged from the cooler at a higher temperature.
In order to remedy the unequal cooling of material particles of different size, it has been proposed to provide means for adjusting the internal mechanism of cooling apparatus to vary the air flow rates. However, it is difficult to realize such adjusting means using screw, pneumatic or hydraulic devices because they must operate in a dusty atmosphere, they must work against the material head pressure and they are subject to heat distortion.
Where adjustment means have been provided in known cooling apparatus, the adjustments have been effected symmetricaly about the central axis of the device. Actual experience with rotary kilns shows that the fine particles of material climb higher in the kiln and discharge at a point which is farther from the center line of the kiln than the point of discharge of the coarse materials. As a result, the coarse and fine material particles do not enter the cooling apparatus symmetrically about the central axis thereof so that, if only symmetrical devices are provided for adjusting the air flow, adjustments cannot be made to provide for equal cooling of these particles of different size.
Finally, the cooling apparatus known in the art is configured to provide either a cross flow of cooling air or a flow which is both counter to and concurrent with the flow of material. Neither of these configurations permits the attainment of maximum cooling efficiency where the ratio of cooling air to material quantity is reduced to a theoretical minimum.